ath9k: use GFP_ATOMIC under spinlock

This is called with spinlocks held so we have to use GFP_ATOMIC.  It's
the sc_pcu_lock in ath9k_stop() that's the issue.  The call tree looks
like this:

ath9k_stop()
ath_prepare_reset()
ath_stoprecv()
ath_flushrecv()
ath_rx_tasklet()
ath9k_dfs_process_phyerr()
pd->add_pulse() => dpd_add_pulse()
channel_detector_get()
channel_detector_create()
pri_detector_init()

channel_detector_create() uses GFP_ATOMIC as well.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Tested-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Acked-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>